1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, in particular, to a circuit for eliminating impact ionization in a differential pair used in an output stage.
2. Discussion of the Prior Art
State-of-the-art CMOS technology offers high density circuit elements, thus making it possible to implement complex functions, analog as well as digital, in modest chip area. Nevertheless, existing CMOS processes have certain disadvantages. One major disadvantage is the low voltage specification that limits the drain-to-source voltage V.sub.ds of N-type MOS transistors. As the voltage V.sub.ds exceeds the allowable level, the total drain current of the device increases substantially due to the contribution of the above-normal drain-to-bulk leakage current. This phenomenon is known as impact ionization and can cause damage to the device.
In a differential pair in an output stage, this limitation has required complicated modifications of the basic circuit to eliminate the problem.